The present invention relates to a method and apparatus for embossing a pattern upon a continuous web of paper, as the web passes between a pair of opposed metal embossing rolls.
1) Field Of The Invention
This invention relates to method and apparatus for embossing a pattern upon a continuous web of paper, typically used for paper napkins, toilet tissue, and the like, by passing the web between a pair of matched pattern embossing rolls. Typically, in such apparatus and methods, as the web passes through the nip, between the rolls, debris is forced out of the paper, and tends to stick to the embossing roll surfaces, shortening roll life.
By the invention, as disclosed herein, an improvement in such apparatus and method is taught, whereby the resulting debris residue is inherently cleaned from the apparatus simultaneously, as the web of paper is embossed.
2) Description Of The Prior Art
Embossing rolls are cylindrical rolls, mounted tangentially parallel and spaced apart to form a nip, therebetween, at the line of roll tangency.
Prior art rolls, are typically of the same diameter, and are rotated inward towards the nip at the same rotational speed. At a point in the nip, equidistant from the center of each roll, embossing projections, extending outward from the embossing roll, and concave imprinting cells, within the opposing imprint roll, move at the same linear speed. The embossing roll, with its embossing projections radially extending from the outer surface, is rotated in timed relationship, with the opposing imprint roll, whereby the embossing projections mesh with the embossing imprint cells, on the opposing imprint roll.
The paper web, as it passes through the nip, is tamped into the imprint cells, by the embossing projections. Tamping the web into the cells, by the tightly fitting projections, embosses a pattern into the web. Embossing paper webs, using a pair of mated steel rolls, is a production technique used to manufacture toilet tissue, paper hankies, paper napkins, paper towels, and like products.
The paper webs, now being received by converters, for embossing, are generally made from recycled paper and inherently contain adherent debris. The debris in the paper generally comprises a mixture of cellulose, latex, and tar. The cellulose, latex, and tar is adherent and sticky.
The presence of such adherent debris, in the recycled paper webs, shortens roll life because of an inherent debris build-up, which occurs within the imprint cells during the embossing operation.
Machine operational speeds are becoming faster, and paper web widths have increased beyond those addressed by prior art. New problems have begun to occur with the increasing speed of paper web embossing machines employing mated steel embossing rolls and using recycled paper webs with their adherent debris content.
When an imprint embossing cell fills with debris, there is no longer clearance between the male embossing projection, and the imprint cell. Thus the web is no longer tamped into the imprint cell, as desired, but is pounded between the male embossing projection and the debris build-up within the imprint cell. The pounding of the embossing projections against the hard debris accumulated within the cell, peens the embossing projections. The peened embossing projections, act to widen the cells as the peened embossing projections pound against the cell walls. A typical symptom of debris build-up, is tearing of the web during the embossing operation; another symptom of debris build up, is poor definition of the pattern embossed into the paper. When the web tears, or the pattern is no longer sharp, the mated rolls must be replaced.
One common, prior art, method used to extend roll life is to merely increase the clearance between the embossing projection and the imprint cell. Thus the embossing projection, and the matching cell is typically etched to create more clearance whereby the cell may hold a greater amount of debris before replacement of the rolls becomes necessary.
Periodically scrubbing the imprint cells to remove the adherent debris, has also been tried in the prior art. However, the debris becomes as hard as glass as a result of the hammering of the embossing projection into the imprint cell and has proven difficult to remove.
Brush rolls, mounted tangent to the imprint rolls, and bearing against the imprint cells, have also been tried in the prior art to continuously clean the imprint cells during the embossing operation.
3) Related References
A prior art patent search has been conducted which resulted in the discovery of the following related references:
a) Methods for embossing paper, to form napkins, paper towels, and toilet tissue is taught in Walton, U.S. Pat. No. 2,729,267, Walton discloses an embossing device wherein a decorative embossed pattern is formed about the periphery of a paper product, such as a napkin, thereby leaving a smooth center area.
b) An embossing machine consisting of an embossing roll and a matching imprint roll is disclosed in Palmer Et Al. U.S. Pat. No. 3,323,983. Palmer addresses the problem of adherent debris build up by providing the imprint roll with open ended circumferential grooves, into which the embossable paper is pressed by male projections on the embossing roll. The open ended grooves permit accumulated debris therein to fall from of the grooves as the imprint roll, upon which the grooves are carried, rotates, during the embossing operation. Alternatively, Palmer teaches the use of a system of brushes, attached to the embossing roll, to clean the grooves of the imprint roll as the rolls rotatingly separate.
c) Nystrand, U.S. Pat. No. 3,867,872, teaches use of a wheel and anvil, to bond and emboss paper products.
d) Schulz, U.S. Pat. No. 4,927,588, discloses embossing a pattern into a multi-ply, continuous paper web, by using a pair of opposed embossing rolls. After embossing a multi-ply web, the plies are separated from one another and longitudinally displaced, relative to one another, and recombined into a multi-ply sheet with the embossments out of register with one another. The device as taught in Schulz incorporates a steel embossing roll, and a rubber roll as the platen.
e) Houk, U.S. Pat. No. 5,158,523, teaches use of a steel embossing roll, as in Schulz, U.S. Pat. No. 4,927,588, rollingly engaging a matching imprint roll having a resilient, impressionable surface and forming a nip therebetween. Projections, in the form of truncated pyramids, on the perimeter of the embossing roll extend outwardly therefrom and imprintingly engage the impressionable surface of the imprint roll. A web passing through the nip, between the embossing roll and the imprint roll, is embossed as the pattern of truncated rectangular pyramids is forced into the impressionable surface of the imprint roll.
In an alternate embodiment, Houk teaches a steel, non-impressionable, imprint roll having a plurality of pyramidal cavities machined into the roll surface wherein he truncated rectangular pyramids, on the embossing roll, tamp the web into the pyramidal cavities at the nip.